Polyester filaments have been commercially available for several decades, and are widely preferred for many apparel and industrial end uses. Practically all of this commercial production has been of poly(ethylene terephthalate) filaments, sometimes referred to as 2G-T, although other ethylene terephthalate polymers have been suggested and some have been made and used commercially.
For many existing high strength uses, sometimes referred to as industrial or technical to distinguish from uses in normal apparel, the polymer has been of high relative viscosity. Such viscosity is measured herein as LRV. Such high LRVs have been about 25 or more; this has also been referred to as a high intrinsic viscosity in a range of about 0.7 to 1.2.
Most commercial production of polyester filaments has been generally of multifilaments, i.e. continuous multifilamentary yarns or spun yarns (spun for instance on the cotton system, from cut staple). These have been of relatively low dpf (denier per filament), generally in a range of about 1 to 6 dpf. This has been so even for industrial end uses, such as reinforcing tires, when high strength has been required. In contrast to multifilaments of low dpf, a small amount of higher dpf filaments have been prepared and used as single filaments (monofilaments), e.g. for bristles and for weaving into screens for papermaking machines and other specialized uses. These, however, are believed to have been made from polyester of regular viscosity, in practice.
When high viscosity polyester multifilamentary industrial yarns have been used to reinforce rubber in tires, for example, they have been twisted and plied into cords. The actual strength contributed by the reinforcing cords in the final rubber articles has been considerably reduced from the higher values measured on the earlier individual low denier filaments and yarns. In other words, measurement of such properties on such earlier low denier filaments and yarns can give a misleading impression of how much reinforcement will later be provided by the cord in a tire, especially after heat treatments that are conventional during tire manufacture. All this has long been well known.
So, it has long been desired by some researchers to make satisfactory polyester filaments of high strength of much higher denier, e.g. of one thousand to several thousand denier (i.e. a dtex of the order of about 1000 to about 10,000 or more dtex on the metric system). These higher deniers are generally intended to be sufficiently high that the single filaments are usable for rubber reinforcement as such, instead of lower denier multifilaments that need to be twisted or otherwise combined into yarns & cords. So these higher denier filaments are referred to as monofilaments, in contrast with multifilaments. It has been particularly desirable to make reinforcing monofilaments from polyester of high viscosity, for use in tires, and for other reinforced articles. In practice, so far, previous efforts have not succeeded in providing commercially acceptable reinforcing monofilaments of high viscosity polyester and/or processes for their production, let alone tires reinforced with such monofilaments.
For instance, Conrad et al disclosed polyester monofilaments in U.S. Pat. No. 3,963,678, using a relatively slow process (less than 200 ypm) that was desirably kept slow to obtain the desired birefringence profile, while Morris et al in U.S. Pat. No. 4,098,864 disclosed another relatively slow steaming process that required sufficient residence time for the steam to be effective.
An important object of this invention is to provide a process for making high viscosity polyester monofilaments that is more viable commercially, and several novel process aspects are provided herein, together with product aspects of the present invention, including the resulting new monofilaments, and the tires and other articles that are the ultimate objective of the invention, as well as "dipped cords" that are intermediate products for making tires. The properties of dipped cords of multifilament yarns have generally been used as a guide to performance in tires. So far as the properties of the products are concerned, their ultimate performance in tires is of great importance, and knot tenacity has been a defect in prior products, in this regard.